There are many applications where an optical detector that can perform light activated switching is desirable. For example, high speed image processing, optical image processing, artificial intelligence circuits, and neural networks. Additionally, in some applications, it may be desirable to provide optical control of microwave monolithic integrated circuits (MMIC). The use of optically activated devices makes possible the use of optical fibers instead of metallic cables. Accordingly, optical devices have many advantages such as reduced size and weight, good electrical insulation and immunity to interference.
One such semiconductor optically controlled device is disclosed in U.S. Pat. No. 4,859,965 entitled "Optical Gain Control of GaAs Microwave Monolithic Integrated Circuit Distributed Amplifier" issuing to Paolella et al on Aug. 22, 1989, which is herein incorporated by reference. Therein disclosed is a field-effect transistor (FET) that is optically controlled, and used in an optical gain control circuit for controlling the gain of a GaAs MMIC distributed amplifier.
While FETs have been used in association with optical illumination in semiconductor devices, there are inefficiencies in that there is poor coupling of the light into the active region of the FET. Recently there has been some research proposing the use of indium tin oxide in a gate which is transparent to light. However, the process to fabricate an indium tin oxide gate is not compatible with conventional microwave device processing techniques.
Accordingly, there is a need for an improved optical FET or metal semiconductor field-effect transistor (MESFET) with increased performance and ease of manufacture that can be incorporated into a MMIC.